SCN Unit 2 Physics
Name:__________________ Date:___________
12.6 Measuring the Critical Angle
Purpose: To explore the critical angle of acrylic
Materials: Ray box with single slit
Semicircular acrylic block
Polar graph paper
Procedure:
1. Using the polar graph paper, place the acrylic block at the centre of the paper with its flat edge along the
horizontal centre line. The normal will be the vertical line going through the centre of the acrylic block
(origin).
2. Project a single ray of light through the curved part of the semicircle so that the incident ray goes through
the origin and the refracted ray comes out the flat edge of the block. Position the ray box so that you
produce a refracted angle in air that is larger than the angle of incidence in the acrylic block.
3. Slowly move the ray box to increase the angle of incidence in the acrylic block. Eventually the refracted
ray leaving the block will disappear. Measure the angle of incidence at this point and record it
Observations:
Critical angle of acrylic ____41°______
Analysis:
1
The accepted value for the critical angles for acrylic is 42.2. How does your answer compare?
Our answer of 41 degrees is very comparable to the accepted value of 42.2. Our value is only 1.2
degrees off.
2
What sources of error could account for any differences between your measured value and the
accepted value?
This error could be due to measurement errors when measuring the angle. The ray box sends out a
fairly wide beam of light, a thinner beam would be more accurate. It was also hard to see the
refracted rays when they were close to the medium division line. It is also difficult to determine
exactly when the light switches from going along the medium line to when it internally reflects.
Another error could be that the light might not have been shone exactly at the origin, and the
semicircular piece of acrylic could have been slightly off it’s position.
3
Did partial reflection and refraction still occur when light was travelling more slowly in the first
medium than in the second? Explain.
Yes partial reflection and refraction still occur when light was is travelling more slowly in the first
medium than the second but only in the angle of incidence is less than the critical angle.
Conclusion: As the angle of incidence was increased, the moved further and further away from the normal until
it ran along the two mediums (acrylic and air). At this point the angle of incidence was approximately 41
degrees. This measurement is very close to the accepted values for the critical anlge of acrylic, which is 42.2.
This could be off due to measurement errors and difficulty in determining the exact point that the ray
internally reflected.
SCN Unit 2 Physics
Name:__________________ Date:___________